In 2010, it was reported that one in four individuals will develop hip degenerative osteoarthritis in their lifetime. Total hip arthroplasty (THA) is becoming an increasingly common treatment for hip osteoarthritis with an estimated 12.6 replacements per 1000 individuals over the age of 18 in the United States from 2008-2009. THA has been successful in reducing and/or eliminating the pain associated with hip osteoarthritis because the previous end stage arthritis has been eliminated. However, there are still pitfalls associated with the THA procedure. Improper placement and alignment of the acetabular implant cup caused by misaligned reaming results in higher failure rates. In addition, the femoral stem must be implanted at a precise depth and angle, and with an appropriate neck length to maintain normal kinematics.
During THA, a surgical reamer is used to clear articular cartilage, tissue, and bone from the acetabulum to create an implantation site for the acetabular cup implant. The location and angle at which the reaming is done determines the final location and angulation of the implant. There is a specific angulation tolerance zone in which the implant should be located in order to minimize the chances of dislocation and need for revision surgery. Currently, there is not an adequate way of precisely determining the proper angulation intra-operatively. Generally, the surgeon estimates the angulation based off of pre-operative templating and intra-operative x-rays. The surgeon must also estimate angulation in this way when impacting the implant into the cup. It has been shown that when the acetabular component is implanted outside a specific “safe zone” of anteversion and vertical tilt, dislocation is 4 times more likely. It has also been shown that anteversion of the acetabular cup cannot be accurately assessed during surgery by orthopedic surgeons experienced in the THA procedure (Hassan D M, Johnston G H, Dut W N, Watson G, Dolovich A T. (1998) “Accuracy of intraoperative assessment of acetabular prosthesis placement.” The Journal of Arthroplasty. 13(1), 80-84). It is also known that THA surgeons cannot consistently implant the acetabular cup within this “safe zone” (Digioia A M 3rd, Jaramaz B, Plakseychuk A Y, Moody J E Jr, Nikou C, Labarca R S, Levison T J, Picard F. (2002) “Comparison of a mechanical acetabular alignment guide with computer placement of the socket.” The Journal of Arthroplasty. 17(3), 359-364). It is well-known that acetabular misalignment contributes to the risk of dislocation, incorrect leg length, impingement, pelvic osteolysis, acetabular migration, component wear, and a potential need for revision surgery.
During THA, a cut is made across the neck of the femur in order to remove the femoral head. The location of this cut plays a role in determining the resultant leg length of the patient. Currently, there is not an adequate way of precisely locating the correct cut line. The surgeon estimates where the cut should be made based off pre-operative templating. This often results in leg lengthening or shortening because of an incorrect cut location. When the femoral component is implanted too shallow or too deep, this results in the patient's limb being too long or too short, interrupting the patient's normal gait. Also, when the femoral implant is inserted into the canal, the appropriate angular orientation is not apparent because there is no indication of proper alignment. When the femoral component is implanted with too great of anteversion or retroversion, the frequency of dislocation is increased. This dislocation is due to the impingement of the femoral neck on the acetabular component.
Although computer-assisted design has been applied with some success to procedures for repair of articular surfaces, in particular of the knee, total hip arthroplasty is a much more challenging procedure involving integration of data from several anatomical planes and axes relating to implantation of prosthetics into disengaged ball-in-socket joint structures and then re-approximation of the implanted joint parts to form an integrated operational ball-in-socket joint with precise implant geometry. To the best of the knowledge of the present investigators, pre-operative imaging and CAD has not been applied to achieve a complete operational patient-specific THA system.
There remains a clear need in the art to improve the implant precision and patient-specificity of THA procedures.